This invention relates generally to wireless communication systems. More particularly, it relates to a receiver or transceiver device which periodically measures signal quality in several fixed beams. The device selects the highest quality beams for each subscriber unit, and communicates with each subscriber using the best beams.
Current wireless communication network topology consists of geographical areas called cells, each of which is centrally controlled by a base station. The base station is physically fixed, and is usually located in the center of the cell. Cellular subscribers set up call connections to the base station, which connects them to other subscribers or to wireline networks such as a landline telephone system.
Within the cell the base station performs functions such as resource allocation to subscribers inside its cell, and medium access control of multiple subscribers attempting to make calls simultaneously. Resources available to the base station depend on the cellular network, and can be a set of time slots per periodic frame. (time-division multiple access or TDMA), frequency channels (frequency division multiple access or FDMA), or spreading codes (code division multiple access or CDMA).
When other subscribers use the same slice of resource as the subscriber of interest (such as the same time slot), the base station receives a signal corrupted by the interfering signals, collectively called cochannel interference (CCI). This degrades signal quality and results in a loss in performance. In TDMA systems base stations in adjacent cells are assigned different frequencies of operation in order to avoid CCI. Because frequencies are a limited resource, however, the same frequency is reused a few cells away. Signals transmitted by a subscriber in that far-away cell often cause CCI at the base station.
One way to reduce CCI is to use multiple antennas at the base station. The signals from the subscriber of interest and the CCI usually arrive along different azimuthal angles (the angle traced in the horizontal plane) at the base station. This occurs because, in general, the two subscribers are typically located at different locations relative to the base station. Angles of arrival at the base station are resolved by using multiple antennas, collectively called an antenna array. The signal received at the set of antennas can be weighted and combined so that the array is more sensitive to signals from certain directions (i.e., the array forms a beams in that direction) Once the directions of arrival of the subscriber""s signal and the CCI signal are determined (by using one of many existing signal processing techniques), the CCI signal can be nulled (i.e., the array forms a null in that direction).
Fully adaptive antenna array base stations having a large number of antennas and high computational capability can estimate directions of arrival of the subscriber and the CCI, and then form a beam towards the subscriber while nulling CCI. These arrays also slowly adapt beams over time as the subscriber moves or as objects in the environment move. Such arrays, however, are expensive to implement. Consequently, current base stations often use a simpler and less expensive multiple antenna system called the switched beam system (SBS) to improve signal quality.
A switched beam system includes of a set of fixed beams that partition the horizontal plane (360 degrees) into sectors. Each sector has a certain beamwidth (e.g., 6 uniform sectors would have a beamwidth of 60 degrees each, for a total of 6xc3x9760=360). The received signal quality on each beam is measured by a simple metric such as the power of the signal received on that beam. The signal on the beam with the best quality is then demodulated in standard fashionxe2x80x94it is downconverted to a lower frequency, analog-to-digital converted, and decoded into bits in order to estimate the transmitted signal. A simple SBS suppresses CCI if the subscriber signal and CCI signals are well separated in direction. However, it provides low performance when CCI lies within a beamwidth of the beams of the SBS. Also, in environments with severe multipath, SBS provides low performance.
Multipath occurs in cellular environments because the radio frequency (RF) signal transmitted by the subscriber is reflected from physical objects present in the environment such as buildings. As a result, it undergoes multiple reflections, refractions, diffusions and attenuations. The base station receives a sum of these distorted versions of the signal (collectively called multipath). In most environments the multipath arrives along certain dominant directions at certain time delays and with certain attenuations. The spread of the signal along directions is called angle spread and the spread in time is called delay spread. When the subscriber is moving, or objects in the environment are moving (such as vehicles), the signal spreads in frequency, which is called doppler spread.
When the multipath is relatively benign, these directions and delays are not resolvable (angle and delay spreads are negligible), and the signal from a given subscriber appears to arrive from one direction only. In that case, SBS performs well if the subscriber signal and the CCI signal are separated in angle by at least one beamwidth. The SBS beamwidth can be made narrow enough so that signals from different subscribers are far apart in angle with a high probability.
When multipath is severe, both the subscriber signal and the CCI signal may arrive along multiple dominant directions. In that case, these directions may lie within a beamwidth with higher probability. A SBS using only received power as a metric may pick a beam with high CCI power and low signal power instead of a beam with high signal power and low CCI power. In order to help the base station differentiate among subscriber and CCI signals, a known portion of the signal called the color code is intermittently transmitted by all subscribers.
The base station randomly assigns unique color codes to all subscribers within its cell when the subscribers register upon entering the cell. Even across cells the color codes corresponding to the subscriber of interest and the CCI subscribers are different with a very high probability. In order to differentiate subscribers by their color codes, the base station must partially demodulate the signal received on each beam. The chunk of the partially demodulated received signal corresponding to the color code is then correlated with the color code assigned to the subscriber. A high value of the color code correlation implies low CCI on that beam. Therefore high signal power and high color code correlation on a beam means good signal quality.
Partial demodulation requires that the signal be downconverted to a lower frequency and then analog-to-digital converted. The device used for downconversion is called an RF chain, and is one of the more expensive components at the base station. In order to reduce costs, the SBS does not simultaneously downconvert signals received on all beams and compute their color code correlations. It only measures the signal powers on all beams, which can be done with the analog signal (before downconversion). This avoids extra costs of installing RF chains for each beam, and one RF chain can be used to downconvert the beam with the highest signal power.
Prior art SBS systems have poor performance in environments with severe multipath. It would be an advance in the art of wireless communications to provide an enhanced switched beam system that provides robust performance in difficult environments. The enhanced system must provide high quality communication links in the presence of thermal noise, angle spread, delay spread and CCI. Particularly, it would be advantageous to provide increases in network capacity and range coverage while reducing susceptibility to multipath and thermal noise.
Accordingly, it is a primary object of the present invention to provide enhanced switched beam wireless communication systems that:
1) has a relatively high tolerance to thermal noise, fading, intersymbol interference, and cochannel interference compared to prior art systems;
2) provides relatively high network capacity and range coverage compared to prior art systems; and
3) has a relatively low cost, a minimum number of components and has a relatively simple construction.
These and other objects and advantages will be apparent upon reading the following description and accompanying drawings.
These objects and advantages are attained by a wireless communication system having, among other components, a beam former for receiving signals from a number of antennas. The beam former combines received antenna signals to form N beams. The N beams are distinct, which in the present specification means that the beams have different angular sensitivities (sensitivity as a function of angle) and/or different spatial location. The system has at least two beam selector switches. Each beam selector switch selects exactly one of the N beam signals. Each of the beam selector switches selects a different beam signal. The system also includes a signal quality measurement device for measuring the quality of each of the N beam signals, and a computer for receiving signal quality measurements from the measurement device. The computer compares the signal quality measurements and commands the beam selector switches to select the highest quality beams. RF chains (exactly one for each beam selector switch) receive the highest quality beam signals. Each selected beam signal is preferably frequency downconverted and A/D converted by the RF chains. Finally, the system includes a signal combiner for combining the converted signals from the RF chains. This system provides a high quality, robust signal for communication.
The N distinct beams preferably comprise a single omnidirectional beam and Nxe2x88x921 directional beams. Preferably, the number of beam selector switches and RF chains is 2, 3, or 4.
Preferably, the system includes a quality measurement switch in communication with the measurement device for selecting which beam is measured by the measurement device. The system may also have a clock for regularly switching the beam selector switches and the QM switch.
The system may also include transmission electronics for transmitting over the identified best beams. Also preferably, the system is a TDMA system. The antennas may be omnidirectional antennas, sectored (directional) antennas, or a combination of omnidirectional and sectored antennas.